Rodriguez B, Blas J, Lorenzo RM, Fernandez P, Abril EJ. Statistical perturbations in personal exposure meters caused by the human body in dynamic outdoor environments. Bioelectromagnetics. Nov 30, 2010. Ahead of print.
Personal exposure meters (PEM) are routinely used for assessing exposure to radio frequency (RF) electric and magnetic fields (EMF). However, their readings are subject to errors associated with perturbations of the fields caused by the presence of the human body.
The objective of this study was to create a novel analytic method for emulating PEM with and without approximating the shadowing effect of the human body.
The experimental environment included several buildings in an open area at the Miguel Delibes Campus of the University of Valladolid. Experimental measurements were conducted at different locations during a 35 minute walk around the campus using a PEM DSP 090 exposimeter with a sampling rate of 3s. The PEM was carried by a volunteer in a small backpack centered on the back at a height of 1.40 m or by dragging a dolly, which was set at the same height where the individual normally carries the exposimeter, in order to provide PEM measurements in the absence of the body. The total measurements were then compared with a 2.5D ray-tracer based on image theory. PEM measurements were emulated, with and without an approximation of the shadowing effect and the Global System for Mobile Communication mobile phone frequency band was chosen for comparison.
The results of this study demonstrated a good agreement in terms of root mean square error and electric-field cumulative distribution function (CDF). A significant improvement was observed when the shadowing effect was taken into account (P-value of 0.05 for the shadowing effect versus a P-value of 10-14 when the effect was ignored). In addition, although electric-field levels in the absence of a human body have been found to follow a Nakagami distribution, a lognormal distribution fit the statistics of the PEM values better than the Nakagami distribution in this experiment.
Interpretation and Limitations
This paper provides insights into the statistical perturbations associated with the influence of the human body on PEM. The demonstrated techniques allowed for the identification and approximation of samples impaired by the shadowing effect in an outdoor environment and showed good agreement with PEM measurements in terms of statistical accuracy. One limitation of this study is that realistic simulated CDFs could not be exactly found by means of an averaged correction factor especially when the average error in the band was employed for correction. Future work in this area will require generalization of the ray-tracing algorithm to represent exposure in the whole body and cover different morphologies.
This study suggests that although the mean could be adjusted by using correction factors, there are also other changes in the CDF that require particular attention due to the shadowing effect.